All commercial brushless, permanent magnet direct current motors known to date operate from external variable voltage, variable frequency sources. The air gap fields produced by permanent magnets cannot be externally controlled such that back electromotive forces (back-emf) of stator windings are strictly functions of speed. As the back-ends of the motor windings increase with rotational speed, winding currents and, thus, torque capacities decrease. Then the source voltage must be increased to force current against back-emf in order to produce the desired torque.
The amplitude of the air gap field in a PM motor is practically constant under normal operating conditions. As the rotor speed increases, so does the back-emf of the motor windings. Consider the following relationships: EQU E.sub.dc -i R.sub.w -L.sub.w di/dt-E.sub.bemf =0, where E.sub.dc is supply voltage, E.sub.bemf is back-emf
voltage, i is winding current and R.sub.w and L.sub.w are winding resistance and inductance. Disregarding the inductance term for steady state conditions and solving for winding current: EQU i=(E.sub.dc -E.sub.bemf)/R.sub.w, and considering that motor torque T=Blir where B is air gap
field density, 1 is the length of winding wire coupling the field B, i is winding current and r is the air gap radius. As the rotor speed increases, so does the back-emf voltage resulting in reduction of winding current if the supply voltage is constant. Thus, with constant supply voltage, the torque is reduced as the winding current is reduced with increasing rotor speed. To overcome this limitation in torque, power converters driving present PMDC motors must boost the output voltage to the winding or selectively switch the winding ON in areas where the field coupling is weak, as used in designs that do not have uniform air gap fields. This increases the complexity and robustness of the motor controller and the degree of voltage stress and heat generation concentrated in the controller power semiconductors. The switching components must then have high voltage ratings as well as high current ratings.
It is practically impossible to have perfectly uniform and balanced air gap field intensities and distributions produced by permanent magnets. This condition, combined with the high rates of change of magnetic coupling caused by switching distributed phase windings cause several undesirable parasitic effects. The most objectionable of these effects is torque pulsations or torque ripple. With trapezoidal or asymmetrical air gap field distributions and phase current waves spanning the pole pitch, PMDC machines have large components of space harmonics. Those harmonics induce circulating currents in the rotor and high core losses in the stator. Therefore, undesirable losses composed of hysterisis and eddy currents take place in the rotor and the stator core. To overcome those parasitic effects, technologies involving pulse width modulation, multi-level power converters or power conditioners are used to make the controller outputs closer to sine waves. This limits the utilization of magnetic core circuits in those motors to approximately 60 percent. The following references address the problems cited above for motors driven by external variable voltage, variable frequency power converters:
IEEE Conference Paper, Titled: Performance Analysis of Permanent Magnet Brushless DC Motor, Authors: Miraoui, A.; Lin DeFang; Kauffman, J. M. PA1 IEEE Transactions on Industrial Electronics, VOL 43, No. Apr. 2, 1996, Titled: Identification and Compensation of Torque Ripple in High-Precision Magnet Motor Drives, Authors: Holtz, Joachim and Springob, Lothar. PA1 1994 Institution of Electrical Engineers, Title: Adverse Electrical Phenomena in Rail Traction Using Alternating Current Motors, Authors: Minalescu, D. and Pantelimon, M.